A DC-DC converter is a central element in any audio power conversion system. Many audio power conversion systems are based on Pulse Modulated Amplifiers (PMA's) operating from fixed DC's typically of a relatively high voltage, in order to be able to reproduce the maximal undistorted output power.
A general problem in such a switching output power stage is the Electro Magnetic Compatibility (EMC), caused by the generally high amplitude of the power stage output PWM signal. This implies high levels of EMI from the switching power stage and associated circuitry, that generally require filtering on all interface terminals and maybe shielding. This increases system cost, and complicates the development and approval of such systems.
A general Pulse Modulated Amplifier (PMA) operating from a high supply voltage will also create pop-noise when turned on. The pop-noise level is proportional to the amplitude of the supply voltage, which implies high levels of pop-noise in most commercial PMA's.
Further, in a conventional PMA, the modulation depth duty cycle is lower at attenuated levels of the input signal, implying that the amplitude of the demodulated signal (i.e. the low pass filtered amplifier output) will be low, compared to the amplitude of the power stage output pulse signal. The efficiency of a power stage generally declines when the duty cycle is lowered. This effect is a result of a decreasing ratio between the amplitude of the demodulated output signal and the amplitude of the power stage output PWM signal.
A higher duty cycle combined with lower amplitude of the PWM signal can retain the same demodulated signal, but will increase the efficiency, lower EMI related problems, and improve the dynamic range at low modulation indexes.
A system capable of changing the DC level is shown in the applicant's international patent application WO03/055059, entitled “Attenuation control for digital power converters”.
Another digital amplifier system including attenuation control is described in U.S. Pat. No. 5,898,340. However, this system includes a complex power stage voltage supply, with an output voltage variable in a wide range. A system without any control system applied to the amplifier will require a linear power supply making it very complex and expensive. The mentioned system also includes A/D conversion means in the feedback path from the analog output. This will increase the complexity and further limit the performance of the system.